CRANKSHAFT

The crankshaft is the part of an engine which translates reciprocating linear piston motion into rotation. The crankshaft has a linear axis about which it rotates, typically with several bearing journals riding on replaceable bearings with the main crank bearings housed in the crankcases main bearing caps. As the crankshaft undergoes a great deal of sideways load from each cylinder in a multi cylinder engine, it must be supported by several such main bearing web housings. High performance engines often have more main bearings and extra bolts for added insurance.

The distance the axis of the crank throws from the axis of the crankshaft determines the stroke of the crankshaft which means distance the piston travels from bottom dead centre (BDC) to top dead centre (TDC), thus determining engine displacement. A common way to increase the low speed torque of an engine is to increase the stroke, often referred to as ‘stroking’. In compensation, it improves the low speed operation of the engine, as the longer intake stroke through smaller valves results in greater turbulence and mixing of the intake charge.

The engine design or configuration and number of pistons in relation to each other and the crankshaft itself leads to a straight in-line, V-configuration or flat-engine design. Production V8 engines use four crank throws spaced 90° apart, high-performance V8 engines often use a ‘flat’ crankshaft with throws spaced 180° apart. For certain engines it is necessary to provide counterweights for the reciprocating mass of each piston and connecting rod to improve the engine balance. These counterweights are cast as part of the crankshaft and while counter weights add a considerable amount of weight to the crankshaft, it provides a smoother running engine and allows higher revolutions to be attained.

Crankshafts can be forged from a steel bar usually through forging or cast in steel. Vehicle manufacturers favor the use of forged crankshafts due to their lighter weight, more compact dimensions and reliability. With forged crankshafts, vanadium microalloyed steels are mostly used as these steels can be air cooled after reaching high strengths without additional heat treatment. Carbon steels are also used, but these require additional heat treatment to reach the desired properties. Cast iron crankshafts are today mostly found in standard production engines where the loads are lower.

Crankshafts can also be machined out of a billet. These crankshafts tend to be very expensive due to the large amount of material removal which needs to be done by using lathes and milling machines, the high material cost and the additional heat treatment required. However, since no expensive tooling is required, this production method allows small production runs of crankshafts to be made without high costs.

Most production crankshafts use induction hardened bearing surfaces since that method gives good results with low costs. It also allows the crankshaft to be reground without having to redo the hardening. Nitridization is is a hardening process used by crank builders that it can be done at low temperatures, it produces a very hard surface.

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